A cationic polypeptide has been isolated from normal human serum that induces contact-inhibited cells to synthesize DNA and divide. The molecular weight of this cationic polypeptide was estimated at about 13,000 daltons and its isoelectric point at 9.7. This polypeptide was shown to be free of insulin-like activity, and stable to heating at 98 degrees C for 20 minutes; its biologic activity was destroyed by incubation with mercaptoethanol, trypsin and chymotrypsin. A similar cationic polypeptide inducing DNA synthesis and mitosis has also been isolated from human pituitary gland extracts. This human pituitary polypeptide exhibited properties identical to those of the human serum polypeptide. Its molecular weight was estimated at about 13,000 daltons and its isoelectric point at about 9.7. The serum cationic polypeptide has been sucessfully radioiodinated with iodine-125, and an antiserum to it developed in a rabbit. Whole human serum, human pituitary and platelet extracts with growth factor activity were shown to crossreact with the rabbit antisera against the serum cationic polypeptide. This indicates a close similarity between the growth factors present in human serum, human pituitary extracts and human platelet extracts. Fetal calf serum or calf serum did not cross-react with the antisera against the human serum growth factor, indicating a species specificity. This work, which is now in progress, allows the development of a radioimmunoassay for the determination of the growth factor in whole human serum and human pituitary and platelet extracts, and in patients with cancer. BIBLIOGRAPHIC REFERENCES: Antoniades, H.N., Stathakos, D., and Scher, C.D. Isolation of a cationic polypeptide from human serum that stimulates proliferation of 3T3 cells. Proc. Nat. Acad. Sci. 72, No. 7, pp 2635-2639, July 1975.